1. Field of the Invention
The present invention relates to a vibration damping device disposed between components to be coupled in a vibration damping manner, and more particularly to a vibration damping device suitable for use in an automotive engine mount, body mount, or the like.
2. Description of the Related Art
Vibration damping devices that involve a rubber elastic body disposed between a first mounting member and a second mounting member respectively mounted on components to be coupled in a vibration damping manner enjoy widespread use in a variety of fields. One known type of device, disclosed in JP-A-9-66721 for example, is a device of a structure comprising a rubber elastic body of generally frustoconical shape overall, with the cylindrical section of the second mounting member affixed to the outer peripheral face on the large diameter end thereof, and with the first mounting member affixed to the center section thereof which includes the small diameter section, whereby the first mounting member and the second mounting member are elastically coupled directly by the rubber elastic body.
Vibration damping devices of this kind can advantageously ensure the volume of the rubber elastic body, and can prevent creation of excessive strain, due to deformation of the rubber elastic body being constrained by the second mounting member. This arrangement makes it possible for the devices to advantageously achieve high load bearing performance and endurance, particularly against load input in the direction forcing the small-diameter end face and the large-diameter end face of the rubber elastic body into proximity with one another. Thus, the application of such devices to engine mounts and other arrangements involving heavy initial load is under study.
With vibration damping devices of conventional design comprising such a rubber elastic body of frustoconical shape, when the rubber elastic body is subjected to load in the rebound direction in the opposite direction from the initial load, forcing the small-diameter end face and the large-diameter end face thereof apart from one another, unavoidably the rubber elastic body is subjected directly to tensile stress. Thus, the rubber elastic body is susceptible to cracking or the like due to input load in the rebound direction, posing the risk of appreciably diminished endurance.
Other known vibration damping devices of this kind are sealed fluid type vibration damping devices like those disclosed in JP-A-8-128493 and JP-A-2002-81491, for example, having a non-compressible fluid sealed in a fluid chamber a portion of whose wall is constituted by the rubber elastic body, with vibration damping action derived on the basis of resonance or other such flow action of the non-compressible fluid.
However, in such sealed fluid type vibration damping devices, in addition to the problem discussed previously, there is the further problem that since the fluid sealed in the fluid chamber is the non-compressible, when the rubber elastic body is subjected to a large load in the rebound direction, the sealed fluid exerts constraining force on the rubber elastic body in the direction hindering elastic deformation, so that vibration damping performance is diminished.
Additionally, where the rubber elastic body is subjected to a large load in the rebound direction, a high level of negative pressure occurs within the fluid chamber, and there is a risk that dissolved air present in the sealed fluid will separate and form bubbles, as well as the risk of noise and shock occurring as the bubbles form and disappear. Additionally, there is a risk that due to bubbles forming in this way becoming compressed during input of vibration, pressure changes within the fluid chamber will not be produced effectively, so that the intended vibration damping characteristics are not obtained.